1. Field of the Invention
The present invention relates to an oxygen concentration detection system and to a vehicle control system which includes the same so as to detect the air-fuel ratio of an internal combustion engine such as a gasoline engine or a diesel engine. More particularly, the present invention relates to an oxygen concentration detection system which can report anomalous conditions of a plurality of types without an increase in the number of signal lines, as well as to a vehicle control system including the same.
2. Description of the Related Art
In a known vehicle control system for an internal combustion engine such as a gasoline engine or a diesel engine, an oxygen sensor is disposed in an exhaust system, and the quantity of fuel supplied to the engine is feedback-controlled in accordance with the sensor output such that the air fuel ratio approaches a target value, whereby CO, NOx, HC, etc. in exhaust gas are reduced.
Know oxygen sensors used in such feedback control include a λ sensor whose output abruptly changes in the vicinity of the theoretical air fuel ratio, and a full range air-fuel-ratio sensor whose output continuously changes from a lean region to a rich region. The full range air-fuel-ratio sensor is advantageous in that it can improve the accuracy of the feedback control as compared with that of the λ sensor.
The full range air-fuel-ratio sensor includes two opposed cells formed of an oxygen-ion conductive solid electrolyte. One of the cells is a pump cell which pumps oxygen out of a clearance (measurement chamber) between the cells and pumps oxygen into the clearance. The other cell is an oxygen-partial-pressure detection cell which produces a voltage corresponding to the difference between a reference oxygen concentration and the oxygen concentration in the measurement chamber. These cells are controlled by means of a control circuit. In the full range air-fuel-ratio sensor, the pump cell is operated such that the output of the oxygen-partial-pressure detection cell is maintained constant, and the oxygen concentration is detected on the basis of the magnitude of current flowing though the pump cell. The operation principle of the full range air-fuel-ratio sensor is described in detail in Japanese Patent Application Laid-Open (kokai) No. 62-148849 of the present inventors.
Meanwhile, an anomaly detection method for an air-fuel-ratio sensor adapted to detect whether or not the air-fuel-ratio sensor operates normally is disclosed, for example, in Japanese Patent Application Laid-Open (kokai) No. 3-272452, also of the present inventors, entitled “Anomaly Diagnosis Method for Air-Fuel-Ratio Sensor”.
3. Problems Solved by the Invention:
However, in JPA No. 3-272452, since an anomaly is judged on the basis of a variation range of voltage of an oxygen detection cell, the cell is judged to be anomalous even when the voltage temporarily exceeds the range as a result of, for example, noise in the control circuit. Therefore, the anomaly judgment may be erroneous. In order to eliminate the noise problem, a signal line for reporting the occurrence of an anomaly may be added. In this case, the wiring becomes complex, and an input port or the like must be added on the side where a report of occurrence of anomaly is received, leading to increased cost. Moreover, when only one signal line is provided for reporting the occurrence of an anomaly, only simple information can be provided.